Monday, May 17, 2010

Bounds, climate sensitivity, and costs of climate change

Hot on the heels of our paper (which is still languishing in the publishing queue, though published on-line) I was rather surprised to come across another paper recently talking about upper bounds on climate sensitivity, and the costs of climate change. It is open access, so you can all read it for yourselves. The authors consider the "long tail" of possible temperature change and how this influences the economic analyses of climate change. They point out that the pathology of Weitzman's result vanishes if an upper bound on climate sensitivity is imposed. They use a Cauchy distribution for sensitivity, and show that the optimal climate policy is fairly insensitive to where this bound is placed, within the range tested of 20-50C. However, they don't appear to justify why these bounds should be used, rather than (say) 500C or 500,000C, at which point the results would probably be rather different.

Though the authors appear to not know about our Climatic Change paper, they actually do cite two of our other papers, in a way that I'm not really enthused by. They interpret us as explicitly ruling out a value for sensitivity greater than 8C, where in fact all of our results are probabilistic and do not arrive at an absolute value (other than any assumed in the prior). But this is only by way of a throwaway comment at the end of their paper, and isn't in any way central to their argument.

Coincidentally, Myles Allen and co are also going on again about how the Jeffreys' Prior solves all the problems of subjectivity (see here for previous). The whole enterprise appears to be a dead end to me and as far as I can tell, they haven't actually demonstrated any practical results, but maybe when he has eliminated all other possibilities he will reluctantly come around to embracing the standard Bayesian interpretation of probability. At least while he is presenting abstruse technical notes on the Arxiv he isn't causing more trouble elsewhere, and it must now be increasingly difficult for him to defend his previous claims. This could make life a little embarassing for the next IPCC report if people don't start producing climate sensitivity estimates that are not based on the now thoroughly discredited uniform prior...

11 comments:

Anonymous
said...

Personally, my mental interpretation of the Weitzman paper had always been that I didn't think a fat-tailed pdf for CS was likely (at least under a couple thousand ppm CO2 - I realize the sun is warmer than it was, and other things are different too, but if we were that close to a runaway greenhouse I think we would have passed that tipping point sometime in last billion years), but that I did think that a fat-tailed damage distribution was possible. I still think it is highly, highly unlikely that climate change will trigger a civilization collapse... but not impossible. And most of the damage functions used in economic models for climate change are, well, not great. (I was going to say especially for larger climatic changes, but I'm also rather dubious of the model results that show that a small warming is beneficial).

And as far as I can tell, Weitzman's conclusions would be similar for a fat-tailed damage function to what they are for a fat-tailed CS function...

Marcus, IMHO civilization could easily survive something as bad as another PETM. The considerable risk of a collapse is from our collective response to the strain of climate change, and it won't take another PETM to put us in the danger zone.

Steve: Yes, disproportionate reaction to disaster is pretty much the mechanism I was thinking of to collapse civilization (eg, massive climate-change induced migration from Bangladesh or other nation in SE Asia somehow triggers a nuclear war between India + Pakistan). Like I said, highly highly unlikely, but IMO more plausible than a runaway greenhouse.

And yes, it doesn't necessarily need a climate sensitivity at the extreme upper end to trigger such an event.

As for climate sensitivity, the weak anthropic principle can be used to rule out large values and a cloistered expert could do so. However, it would be better to consult geologists who are in ignorance of the instrumental period (easy to find such, I opine).

>"I wouldn't mind seeing what a Jeffreys prior actually looks like in this case..."

I am speculating/guessing so please put me right if you disagree: To be invariate to reparameterisation, it has to be some sort of average.... but average of what? An average of a uniform prior on sensitivity from -200C to -100C and uniform prior on 100C to 200C would sound pretty meaningless.

Some sort of average of plausible priors might be useful but I am not sure I can see how it could be an average of plausible priors only.

That is no reason to change a 'I wouldn't mind seeing what a Jeffreys prior actually looks like'attitude and I would join James in that.

>"I didn't think a fat-tailed pdf for CS was likely (at least under a couple thousand ppm CO2 - I realize the sun is warmer than it was, and other things are different too, but if we were that close to a runaway greenhouse"

I agree high climate sensitivities can be ruled out as a result of reviewing what has happened in history.

This is therefore highly likely to be an irrelevant nit-pick at the above runaway assertion. I am posting just to see the reaction to my nit-pick so feel free to criticise.

If climate sensitivity was high but there is an almost negligable effect of temperature on release of GHGs, you would expect a highly variable temperature record in response to external GHG forcing but it wouldn't really be close to a runaway effect.

So to talk about closeness to a runaway effect you really need to be talking / know about the temperature effects of releasing GHGs not about high levels of climate sensitivity.

The repeating patterns of interglacials with large temperature changes may indicate that the temperature effects of releasing GHGs might be quite variable with circumstances so that we sometimes get temporary/limited runaway effect that results in a change from glacial to interglacial or the reverse. It tends to stop at similar levels suggesting that either:

At higher and lower ends of temperature range, the temperature effects of releasing GHGs become limited. So not much to worry about a significant temporary runaway effect starting in the near future.

Or it could be that the temporary runaway effects tend to end after a certain amount of change in temperature. So this alternative does not rule out a significant temporary runaway effect starting.

We could be close to a temporary limited runaway effect while also being very sure we are a long way from a permanent irreversable runaway effect?

Thanks Carl, I didn't realise that link. I will reserve judgement on how good/bad a Jeffreys prior is, until I actually see one! But I do object in principle to the claim that some cook-book recipe can solve the problem.

That's what I was trying to get at above as well. Lots more runoff from changed precipitation, lots of fertilization in coastal oceans -- is the ocean/biology feedback going to be different when there's already dead zones from excess nitrates, and excess petroleum* for example. Not that I know of anyone who's worked much yet on that kind of feedback, but someone must be.----

* if you watch thishttp://scienceblogs.com/gregladen/2010/05/dee_boersma_pay_attention_to_p.phpyou'll see a slide of areas where there's enough oil floating on the ocean to be a significant ongoing problem coating diving birds and other air-breathers -- and it's an astonishingly large number of big areas of coastal oceans. Mostly from ballast water washing by tankers.

What else is it killing besides those penguins she's talking about? Plankton? Those are the things new this time around for climate sensitivity. Important on the short term? (shrug). I wonder if the biologists even know it might be a useful question to ask.

PS, did you know about the Japanese industry plan to harvest penguins for handbags and golf club decoration? Tsk. Amazing science vid (TED talk)